Endogenous isoquinolines are formed by condensation of biogenic amines—such as phenethylamine—and simple aldehydes, such as formaldehyde or acetaldehyde. They are known to modulate neurotransmission, central metabolism and motor activity. An endogenous TIQ derivative salsolinol (1-methyl-1,2,3,4-tetrahydroisoquinoline-6,7-diol) is considered to be a causative factor of Parkinson's disease (PD), while (R)-1-MeTIQ (1-methyl-1,2,3,4-tetrahydroisoquinoline) was shown to possess an antiparkinsonian activity. Until recently 1MeTIQ was the only known neuroprotective/PD preventing TIQ derivative. In 2006 Katsuhiro OKUDA et al. (Biological and Pharmaceutical Bulletin 29 (2006) pp. 1401-1403) discovered that 5-/6-/7-monohydroxylated 1MeTIQ derivatives are neuroprotective and PD preventing indeed, even more so than the parent compound.
It has been demonstrated that concentrations of many endogenous TIQ derivatives are significantly elevated in the urine and cerebrospinal fluid of PD/ADHD (attention deficit hyperactivity disorder) patients compared to controls, the content of 1MeTIQ however is significantly decreased in PD patients' cerebrospinal fluid and brain.
Salsolinol is formed enzymatically as well as non-enzymatically as a condensation product of acetaldehyde—the primary metabolite of ethanol—with dopamine in the brain of mammals. Salsolinol affects the uptake of catecholamines into nerve terminals, the release of stored catecholamines and the activity of monoamine oxidase (MAO), catechol-O-methyl transferase (COMT) and tyrosine hydroxylase. Ethanol induced elevation of salsolinol levels is known to participate in the development of ethanol addiction/alcoholism.
Salsolinol (SAL) has been postulated to mediate some of the addictive properties of alcohol. A number of studies have shown that primates self-administer SAL even in nanomolar concentrations when intracranially injected to certain brain region. Also acetaldehyde is self-administered when injected intracranially although much higher concentrations are needed. Studies have confirmed that SAL is released during suckling on lactating sheep. It is clear that SAL mediates reinforcing effects on a number of primate species. Studies have shown that controlled amounts of ethanol intake have only miniscule effect on brain SAL levels. Still it is clear that ethanol intake elevates dopamine and acetaldehyde concentrations inside brain and so the concentrations of starting materials for Pictet-Spengler reaction forming SAL are elevated. In this light it seems certain that alcoholics with higher ethanol intake and generally higher aldehyde dehydrogenase (ALDH) activity (see Alcohol Clin. Exp. Res. 2009 November; 33 (11):1935-44) try to compensate lower acetaldehyde and SAL concentrations by increased drinking.
Most tetrahydroisoquinolines penetrate to the brain in pharmacologically relevant amounts and induce a variety of effects. Most of the TIQ and 1MeTIQ exit the brain (90.4% and 95.3%) and is excreted in urine (76% and 72%) unchanged. The hydroxylated (C4 of the isoquinoline backbone) derivatives of TIQ and 1MeTIQ were the most abundant metabolites in the urine (2.7% 8.7%).
In European Journal of Medicinal Chemistry 41 (2006) pp. 241-252 6-fluoro-1-methyl-1,2,3,4-tetrahydroisoquinoline has been reported as a possible agent against Parkinson's disease.
EP 2 090 576 A1 discloses certain 5,8- and 6,7-difluoro substituted isoquinolines as intermediates in the preparation of pyrazolo[1,5-a]pyridines for use as metabotropic glutamate receptor modulators.
It has now surprisingly been found that the substitution of 6- and 7-positions of the isoquinoline backbone blocks the adverse metabolic formation of dihydroxy compounds and thereby also improves the desired activity of the isoquinoline compound in question.